1. Field of the Invention
The present invention relates to a voltage surge damping device for DC electromagnets as well as an DC electromagnet equipped with such a device.
It relates more particularly still to a device adapted for fulfilling at least two functions, namely; a function limiting cut-off voltage surges of the DC electromagnet and a function consisting in destroying the remanent induction of the magnetic circuit of the DC electromagnet when the control coil is not supplied with power.
2. Description of the Prior Art
It is generally known that in some types of electromagnets the function of limiting cut-off voltage surges is provided by a short circuit ring made from a copper alloy preferably mounted on the fixed part of the magnetic circuit supporting the coil of the electromagnet.
Moreover, elimination of the remanent induction of the magnetic circuit when the control coil is not supplied with power is obtained by defining air gaps, in the closed condition of the electromagnet, by means of pieces made from an amagnetic material which may also have damping properties.
These two functions are fulfilled for example in the silient electromagnet described in French patent application No. 84 03014 filed on the 28th Feb. 1984, in the name of the applicant for: "Silent Electromagnet and Contactor using such an Electromagnet" which describes an electromagnet comprising:
a fixed magnetic circuit, in the form of an E, in which the end of the central leg has a V shaped concavity in profile and in which the ends of the two side legs have an external chamfered face,
a mobile magnetic circuit, also in the form of an E, in which the end of the central leg has a convex shape in profile substantially complementary to that of said cavity and in which the ends of the side legs have an inner chamfered face, and
a coil wound about the central legs of the mobile and fixed magnetic circuits.
In this example, the function of limiting the cut off voltage surges is provided by means of a ring made by stamping and bending a metal sheet of small thickness, and mounted on the web of the magnetic circuit.
The air gaps formed in the closed condition of the electromagnet by said concave and convex shapes and by the chamfered faces of the magnetic circuits of the electromagnet are then materialized by molded rings, made for example from silicone coated elastomer, mounted on the two side legs of the fixed magnetic circuit and on which come to bear the ends of the side legs of the mobile magnetic circuit.
However, in this embodiment, these two functions are only correctly fulfilled when centering of the two fixed and mobile magnetic circuits is good and when the guide play of the mobile circuit is small.
In fact, because the lateral or central oblique air gaps must be small, by definition, so as to reduce the air gap reluctances in open circuit, appreciable off-centering and/or too large a guide play cause, during closure, dissymmetry of the lateral air gaps which may lead to magnetic "sticking" on one of the side legs, which may lead to total non closure of the magnetic circuit.
To avoid this drawback, it is then necessary to place on these two faces a piece of amagnetic material, preferably with an antifriction property, which will be limited to a guide function, when closure is beginning and will not provide the function of defining the final air gap (antiremanent air gap) since this function remains provided by the previously mentioned elastomer rings.
It is clear that these arrangements use a multiplicity of parts (voltage surge limiting ring, elastomer rings, antifriction amagnetic material piece).
The purpose of the invention is therefore more particularly to overcome this drawback.